As Nathrao has said, your query is not complete. While the standard shift time is 8 hours, the law states that anyone working more than 9 hours a day is entitled to overtime. This means your employer is allowed to have a 9-hour workday. Additionally, work hours do not include any breaks, so if you have a 30-minute lunch break, your actual working hours are 9 hours.
In any case, what do you mean by how it could be changed? You can inform your employer that they are not following the law and request a reduction in work hours. Alternatively, you can file a complaint with the labor commissioner.
However, be prepared for the possibility that your employer may choose to terminate your employment in response.
"My office working hours are 9 hours and 30 minutes, and the law states that it should not exceed 8 hours. Can you please suggest how this could be changed?"